Many filler alloys have been developed and researched for brazing titanium and titanium alloy. The developed Filler alloys are largely divided into four groups: aluminum (Al)-based filler alloy, silver (Ag)-based filler alloy, titanium (Ti)-based filler alloy, and zirconium (Zr)-based filler alloy. In case of using the Al-based filler alloy and Ag-based filler alloy, a detrimental intermetallic compound phase is formed by reactions between the major elements comprising the filler metals and Ti base alloys in Ti brazed joint area. Thus, they are improper for the formation of a robust Ti joint body which requires heat-resistance, corrosion resistance, strength and reliability. On the contrary, active-element based filler alloys, containing large amount of active elements such as Ti and Zr element have been well known to provide a good Ti brazed joint body having excellent heat-resistance corrosion resistance and strength at room temperature and high temperature. However, since the active element-based filler alloys have relatively high melting points, their brazing temperature can be close to the recrystallization temperature and beta transus temperature of titanium base metals to be joined. So the microstructure of Ti base metal may be altered if the brazing temperature exceeds these restrictive temperatures. Eventually, the change of original microstructure of Ti-based alloy after brazing causes the mechanical properties of base metal to be impaired.
When brazing (α+β) type and β type Ti alloys, there has been actual restriction in selecting a brazing cycle and a filler alloy with a low melting point that can allow heat-treated (α+β) type and β type Ti alloys to braze without changing their heated structure. Theoretically, it is desirable that Ti-based alloy should be brazed at a temperature as lower as from 55 to 83° C. than its beta transus temperature. If Ti alloy is brazed at a temperature higher than the restrictive temperatures, the mechanical properties such as strength and ductility particular in the α+β type and β type Ti alloys heat-treated may be impaired. The damaged mechanical properties of Ti base metals are hard to be recovered without another heat treatment for the brazed assembly. After brazing, the re-heat treatment of brazed part such as quenching may deform the brazed assembly in large size with complicated shape due to the rapid change in the temperature. Therefore, post heat treatment of the brazed assembly after brazing is not preferable to solve this problem.
In order to reduce the impairment of Ti base metal after brazing, using filler alloy with low melting point has been preferable as an effective method. With the active element based filler alloys developed, their brazing temperature has to be set at temperature more than 850° C. This is because a conventional filler alloy having the lowest melting point has a liquidus temperature of approximately 843° C. at Ti-37.5Zr-15Cu-10Ni in wt % (Ti48.5Zr25.7Cu14.8Ni10.6 in atomic %). In case of beryllium (Be) containing active brazing filler alloys such as Zr—Ti—Ni—Be have a liquidus temperature less than 800° C. because Be element is a strong melting point depressant of Ti and Zr. However, the use of Zr—Ti—Ni—Be has been restricted since Be is a hazardous element. Also, another Zr-based filler alloy in free of Be, which is Zr-11Ti-14Ni-13Cu (wt %) (Zr50.3Ti17.0Cu15.1Ni17.6 in atomic %), has a liquidus temperature about at 830° C. Even though its liquidus temperature is lower than that of other Ti-based filler alloys by approximately 20˜30° C., but it has a drawback that brazing should be performed at a high temperature more than 900° C.
Also, conventional filler alloys taking an active element, e.g., Ti or Zr, as a base includes copper (Cu) and/or nickel (Ni) more than 14% by atomic ratio as a melting point depressant. Herein, although Cu is effective in drastically dropping the melting point of an alloy in the presence of Ni, it has a shortcoming that it also deteriorates mechanical and chemical properties of the brazing joint body when it is included abundantly. In short, Cu causes a brittle intermetallic compound to be generated easily during a joining process and a process of cooling a joint body.
According to a study by Botztein et al published in Materials Science and Engineering A, Vol. 206, pp. 14-23, 1995, when a Ti alloy is brazed using a Cu—Ti—Zr filler alloy, a brittle λ-Cu2TiZr Lavas phase appears during slow cooling process. To prevent the formation of the brittle phase, it is recommended to limit the Cu concentration in the joint body to go not more than approximately 10 wt % to approximately 12 wt %.
Also, a study by Chang et al published in Journal of Materials Engineering and Performance, Vol. 6(6), pp. 797-803, 1997, reveals that when Ti-6Al-4V is joined at approximately 960° C. using Ti-21Ni-14Cu molten filler in a joint body reacts with an (α+β)-type Ti-based alloy to thereby form a lamellar-type Widmanstatten structure consisting of a Ti2Ni phase and a Ti2Cu phase. Herein, the Ti2Ni phase disappears as Ni component existing on a joint body in the initial state diffuses into the inside of a base metal during a 2-hour-long diffusion process. On the other hand, the Ti2Cu phase still remains on the joint body.
In consequences, the high melting temperature of conventional filler alloys and a high content of Cu have been pointed out as factors degrading durability of a Ti-brazed assembly. Therefore, when Ti alloy is brazed using typical filler alloy, a short brazing cycle is needed in order to protect base metal from being mechanically damaged. To be specific, the Ti alloy should be heated up to quickly reach as high brazing temperature as approximately 850° C. and up, maintained at the brazing temperature for a short time no longer than approximately 15 minutes, and quenched rapidly. The short brazing cycle may be disadvantageous when a metal like Ti having heat conductivity lower than other metals is used for a joint body.
To sum up, it is necessary to limit the amount of Cu added to a filler alloy to a minimum amount in developing new active element-based filler alloy having a low melting point.